Organic light emitting devices (OLEDs), which make use of thin film organic materials that emit light when subjected to a voltage bias, are expected to become an increasingly popular form of flat panel display technology. Potential applications include cellphones, personal digital assistants (PDAs), computer displays, informational displays in vehicles, television monitors, as well as light sources for general illumination. Due to their bright colors, wide viewing angle, compatibility with full motion video, broad temperature ranges, thin and conformable form factor, low power requirements and the potential for low cost manufacturing processes, OLEDs are seen as a future replacement technology for cathode ray tubes (CRTs) and liquid crystal displays (LCDs). Due to their high luminous efficiencies, OLEDs may replace incandescent, and perhaps even fluorescent, lamps for certain types of applications.
Light emission from OLEDs typically occurs via electrofluorescence, i.e. light emission from a singlet-excited state formed by applying a voltage bias across a ground state electroluminescent material. It is believed that OLEDs capable of producing light by an alternate mechanism, electrophosphorescence, i.e. light emission from a triplet excited state formed by applying a voltage bias across a ground state electrofluorescent material, will exhibit substantially higher quantum efficiencies than OLEDs that produce light primarily by electrofluorescence. Light emission from OLEDs by electrophosphorescence is limited since the triplet excited states in most light emitting organic materials are strongly disposed to non-radiative relaxation to the ground state. Thus, electrophosphorescent materials hold promise as key components of OLED devices and other optoelectronic devices exhibiting greater efficiencies relative to the current state of the art. For example, OLEDs capable of light production by electrophosphorescence are expected to exhibit a reduction (relative to OLEDs which produce light primarily by electrofluorescence) in the amount of energy lost to radiationless decay processes within the device thereby providing an additional measure of temperature control during operation of the OLED.
Improved light emission efficiencies have been achieved by incorporating a phosphorescent platinum-containing dye in an organic electroluminescent device such as an OLED (Baldo et al., “Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices”, Nature, vol. 395,151-154, 1998) and phosphorescent iridium-containing dyes have also been employed (US 2003/0096138). Polymerizable phosphorescent iridium complexes based on a ketopyrrole ligand are disclosed in pending U.S. application Ser. No. 11/504,552, filed on 14 Aug. 2006, which claims priority from U.S. provisional application Ser. No. 60/833,935, filed on 28 Jul. 2006, the entire contents of which are incorporated by reference in their entirety.
Notwithstanding earlier developments, there is currently considerable interest in finding novel phosphorescent materials, which increase efficiency and provide for a greater measure of control of the color of light produced by an OLED, while achieving improved lifetime of the devices.